Do you have an iMac CRT? It looks like
and may come in a variety of colors. If yours is not like this, it is not an iMac CRT, and can't be upgraded to Mac Box Set which included Mac OS X 10.5. Only 867 Mhz and faster Flat Panel iMacs
, Intel iMacs
(most recent Intel iMacs look like this), and PowerPC G5 iMacs
(iMac G5 and Intel first and second generation look like this) can have the Mac Box Set installed. You posted to the iMac G3 forum, so we had to ask.